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Batch MKV to WebM Conversion in Your Browser

Take heavy MKV captures from OBS and turn them into lean VP9 WebM ready for the web.

  • Batch convert MKV (H.264/H.265) to VP9 WebM
  • Container simplification: one video + one audio track
  • Typically smaller than the source at matched quality
  • Local browser processing, no upload, no account
  • Download individually or as a ZIP

Overview

OBS-to-CMS is the workflow this page exists for. OBS captures land as MKV (a flexible container that can carry several audio tracks, subtitle tracks, and chapter atoms), and most CMSes (or your own self-hosted <video> tag) want the opposite: one video track, one audio track, modern codec, lean container. WebM with VP9 fits that brief, with the caveat that the converter only carries the primary video and audio track through.

The video usually moves from H.264 inside MKV to VP9 inside WebM, which often produces a smaller file at matched perceived quality. The container itself also gets simpler, with no chapter atoms, no extra audio streams, and no embedded subtitles.

The conversion runs locally in your browser, so a stack of multi-hour OBS captures works the same as a few short clips. No upload, no server timeout.

Technical notes

Container simplification

MKV can carry many audio tracks, subtitles, and chapters. The WebM output keeps just the primary video and audio track, which is exactly what HTML <video> wants.

VP9 efficiency

Re-encoding from H.264 or H.265 MKV to VP9 WebM often produces smaller files at matched perceived quality, with the largest gains on talking-head and screen content. Exact ratios vary with content.

Sources: MDN: Web video codec guide · Reviewed 2026-10-12

Browser compatibility

VP9 WebM is widely supported in current browsers. Pair it with an H.264 MP4 fallback when older Apple devices or browsers are part of the audience.

Sources: MDN: Web video codec guide · Reviewed 2026-10-12

Good to know

Subtitles and extra tracks dropped

MKV files can hold multiple audio tracks, subtitles, and chapters. The WebM output keeps only the primary video and audio track. Ship subtitles as a separate WebVTT file in your <video> element.

VP9 encode is CPU-bound

VP9 encoding can be slower than hardware-accelerated H.264 encoding. Long OBS sessions can take meaningful time, and actual speed depends on the codec acceleration exposed by the browser and device.

Sources: MDN: WebCodecs codec selection · Reviewed 2026-10-12

How to convert MKV to WebM in bulk

Convert a folder of MKV files to WebM in your browser without uploading anything.

01

Add your MKV files

Drag and drop one or many MKV files into the queue, or click to pick them.

02

Confirm the output preset

Output is preset to WebM. For web embedding, drop the width to 1280 or 1920 and use the balanced quality preset.

03

Start the batch

Click Convert files. Each file is processed locally in your browser.

04

Download WebMs

Download converted WebM files individually, or grab the whole batch as a ZIP.

Frequently Asked Questions

Are my MKV files uploaded anywhere?

No. The conversion runs in your browser via WebCodecs and your files stay on your device.

What happens to subtitle tracks?

They are not carried into the WebM output. If you need subtitles for the web, ship them as a separate WebVTT track in your <video> element.

Will the file actually be smaller?

In most cases, yes. VP9 is more efficient than the H.264 typically found inside OBS-captured MKVs, and the WebM container has less overhead than MKV.

Why not stick with MKV?

Native MKV playback is not a dependable web target. WebM is based on Matroska and is designed for browser playback, though older devices may still need an MP4 fallback.

Sources: MDN: media container formats · Reviewed 2026-10-12

Which browsers does this work in?

Conversion requires WebCodecs video encoding and decoding, but each browser and device supports a different codec subset. MP4, MOV, and MKV output use H.264 video with AAC audio; Firefox and desktop Linux browsers do not natively encode AAC. Safari 16.4 added video WebCodecs, while Safari 26 added audio encoding and decoding. WebM output uses VP9 with Opus and may work where AAC output does not. For the widest support, use a recent desktop Chrome or Edge release, or Safari 26+. The converter still validates the actual source and output codec configuration when a job starts.

Sources: MDN: WebCodecs API · MDN: WebCodecs codec selection · WebKit: WebCodecs video support in Safari 16.4 · WebKit: WebCodecs audio support in Safari 26 · Mediabunny: supported formats and codecs · Reviewed 2026-10-12

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